U.S. Pat. No. 4,440,498 (Sheem) relates to an optical fiber gyroscope with a 3.times.3-couper, or rather directional coupler, which, with the aid of the 3.times.3-coupler, operates close to the quadrature point without the need for a modulator.
U.S. Pat. No. 4,479,715 (Sheem) discloses a fiber gyroscope in which the gyrorate is determined by forming the quotient of the signals of two photo-diodes. This type of signal evaluation has the advantage that the light intensity of the light source is cancelled out of the equation so that the fluctuations of the light source do not have any influence on the measured result. Additionally, the fluctuations in the damping of the fiber coil also cancel each other in the calculation. The same is true for any damping caused by the coil splices.
The above mentioned related application U.S. Ser. No. 07/508,294 discloses a method which permits determining a precise Sagnac-signal in spite of unknown fluctuations of the polarization characteristics of the fiber coil and of the coil splices.
U.S. Pat. No. 4,653,917 (Moeller et al.) discloses a fiber optical gyroscope operating with an unpolarized light source providing a broadband spacially coherent light beam supplied by a super-luminescent diode source. A reduction in base band noise is achieved due to the use of the essentially unpolarized light beam.
It is known that fiber gyroscopes having a 3.times.3-coupler, due to their structural features, basically do not permit an operation with a reciprocal light path. Environmental fluctuations, for example, temperature fluctuations, pressure fluctuations, and so forth, of the polarization transmission characteristics of the fiber coil become noticeable in the form of a zero point drift and in a scale factor drift of the gyro signal. The zero point errors can be avoided by using nonpolarized light. However, the scale factor changes which are caused by contrast variations of the interference signal, remain. In accordance with the teaching in the above mentioned parent application, it is possible to take into account the scale factor variations by evaluating the signals P.sub.1, P.sub.2, P.sub.3 of the three detector diodes D.sub.1, D.sub.2, D.sub.3 in such a manner that the three unknown values of the Sagnac-phase, the contrast, and the intensity of the light source follow in an unambiguous manner.
However, the following problem still exists. In the above mentioned method the three detector signals P.sub.1, P.sub.2, and P.sub.3 are the prerequisite for the signal evaluation for which the respective equation is as follows when nonpolarized light is used: EQU P.sub.1 =v.multidot.D.multidot.I Equation (1) EQU P.sub.2 =v (A+kB.multidot.cos(.PHI.-c))ds.multidot.I Equation (2) EQU P.sub.3 =v (A+kB.multidot.cos(.PHI.+c))ds.multidot.I Equation (3)
In the foregoing equations v is a conversion factor for describing the efficiency of the photo-diodes, I is the intensity of the light source L; and A, B, c, and D are constant coefficients which result from the transmission characteristics of the 3.times.3-coupler. Further, k is the contrast factor of the interfering signal resulting from the polarization characteristics of the fiber coil, and ds is the damping factor which contains the damping of the fiber coil including any coil splices. Particularly, the damping ds must be known because this factor is explicitly required in the signal evaluation of the above mentioned method: ##EQU1## A determination of the Sagnac-phase and thus of the needed gyro rate thus requires that the damping factor ds is known. However, it is a requirement for these gyroscopes that their technical use or useful life must be within the range of ten to twenty years. In such time periods the damping of the fiber coil may change, due to increasing so-called micro-bending and splice losses which change with time. In the case of using such a fiber gyroscope after it has not been used or has been stored for several years, the damping coefficient ds will have changed in an unknown way. An exact determination of the gyro rate is thus not assured anymore.